1. Field of the Invention
This invention relates to temperature compensated circuits, and more particularly, to improvements in temperature compensated bipolar transistor switch circuits.
2. Description of the Related Art
In the past, it has been recognized that the switching voltage of typical bipolar switch circuits is dependent on temperature, and many efforts have been advanced to attempt to compensate for this circuit deficiency. It is generally recognized that switch circuits need to reliably turn on at a predictable input voltage. In addition, the effect of input voltage noise on turn-off and turn-on voltage levels must be minimized. Finally, the turn-off and turn-on voltages themselves must be temperature stable. These considerations are complicated when the input voltage is a differential voltage rather then a voltage reference, for example, to ground.
A typical switch of the prior art with which the invention may be applied is shown in FIG. 1, and denoted by the reference 10 within the dotted box. The switch is fabricated of bipolar devices including a pair of PNP input transistors 11 and 12. A pair of PNP sensing transitors 13 and 14 are connected to the respective emitters of input transistors 11 and 12, a resistor 16 being provided between the base of the sensing transistor 14 and the emitter of the input transistor 12 to provide a voltage turn-on/turn-off hysteresis. An output transistor 18 is connected with its base attached to the collector of the sensing transistor 14 and the emitter of sensing transistor 13. The output current of the switch is derived in the series circuit including a collector and emitter of the output transistor 18. A current bias source 20 is provided in the circuit including the emitter and collector of the sensing transistor 14, as shown. (The current value I.sub.KTQ /2 shown outside the dotted box is not a part of the prior art switch circuit, as will be become apparent from the description below.) In the prior art circuit the biasing currents provided on lines 22, 23 and 24 were essentially temperature independent, that is, had zero variation with respect to variations in the temperature of the circuit. Temperature compensation was accomplished by appropriate biasing of a differential pair of transistors, similar to those shown in the differential pair circuit 40 to which a differential voltage input was applied, as shown. The differential pair includes two transistors, 42 and 43 to the respective bases of which the differential input voltage is applied. The emitters of the transistors 42 and 43 are connected together, and the collectors have respective identical resistors R.sub.g connected between the collectors and ground. The output from the differential pair is derived at the collectors of the transistors 42 and 43, to which base connections are made to the respective input transistors 11 and 12 of the switch 10. In efforts to compensate the switch circuit 10 for variations in ambient temperature, a bias current was applied to the emitters of the differential pair transistors 42 and 43 which varied as a first order function of temperature. In the prior art, it has been considered more desirable to utilize a temperature proportional current to bias the differential pair as opposed to a zero temperature coefficient biasing current since using a zero temperature coefficient bias current, the gain of the differential pair decreases as temperature increases. Nevertheless, using the aforementioned temperature proportional current results in the gain being constant over temperature, but the result is still not ideal.